A pump diaphragm, which forms a part of the dispenser mechanism, is known, for example, from U.S. Pat. No. 4,457,454. In this case, it was a matter of a dome-shaped body made of flexible material, with a central conduit for the product mass as well as a central inlet for the power to push the pump membrane back and forth.
A dispenser of the general type with which the present invention is concerned is disclosed in DE-OS No. 34 35576. There the piston, which is sucked upward during the course of the progressive reduction of the volume of pasty product before it, has a closable air-venting nozzle. Through this nozzle the air trapped between the surface of the product and the wall of the piston, during the insertion of the same, is able to escape. The filling operation is carried out with the dispenser in an inverted position, with the head pointing downward. A reclosing is needed in order to make it possible to generate the partial vacuum within the housing necessary for the later operation of the dispenser mechansim. In the case of the known solution the closure of the air-venting nozzle is accomplished by means of a stopper or plug. But due to the fact that there must be sufficient adhesion to hold the stopper in place, or even because some sort of snap-locking arrangement is necessary, the product mass is subjected to considerable stress. This can lead to an unwanted squeezing out of the same, for example, in the case of an improperly closed discharged nozzle. In the effort to improve the manageability of the rather small stopper, the latter is equipped with a large, frequently troublesome handling lug in the form of a disk.